1. Field of the invention
The invention concerns an installation for spraying coating product, notably an electrically conductive coating product such as a water-soluble or metallic paint applied electrostatically; it is more particularly concerned with a new arrangement for changing such products rapidly. The invention finds a particularly beneficial field of application in areas where the objects to be coated succeed each other along a common spraying location and where any two consecutive objects have generally to be covered with coating product of different colors. Operating constraints of this kind requiring extremely rapid color change cycles are encountered in the automobile industry in particular, for painting bodywork members. The invention is more particularly directed to solving the additional problems which arise in this context from electrostatic application of water-soluble and some metallic paints having a relatively high electrical conductivity.
2. Description of the prior art
A paint spraying installation in a production unit as large as an automobile manufacturing plant, for example, generally comprises several closed loop paint circuits which are very long (these circuits may cross an entire section of the plant) and which establish the connection between large paint storage tanks and the various spraying booths. It is therefore necessary to provide a circuit of this kind for each color and another circuit of the same kind for the solvent or cleaning product. For obvious safety reasons these circuits are electrically grounded.
In a paint spraying booth supplied in this way the objects to be painted, which are automobile bodies in the example under consideration here, are carried by conveyor means which pass through the booth in which are electrostatic spraying means operative within a specific "activity area" within the booth.
These spraying means may be "bowls" rotating at high speed or pneumatic or hydrostatic sprayers. A routine problem to be solved in this type of installation is that of changing color between two consecutive objects. In the automobile industry in particular there is no question of painting bodies in series the same color. To the contrary, the usual situation is that the color has to be changed virtually every body. This implies the ability to carry out extremely rapid spraying means rinsing and drying cycles. To give an example, a color change may take place about every minute and the time allowed for carrying out all the operations necessary for changing color may be in the order of 10 seconds.
In known conventional installations all the coating product circuits, a compressed air circuit and a cleaning product circuit are connected via isolating valves to a manifold having a common outlet branch connected to the sprayer. This arrangement will be referred to hereinafter as a "coating product change unit" or, more simply, a "color change unit". Conventionally, to change color the valve on the circuit of the coating product in use must be closed a particular (computed) time before the end of the current spraying phase and then the compressed air valve opened to propel the remaining product towards the sprayer. A cleaning cycle is then carried out comprising a succession of injections of cleaning product and compressed air until the pipes are clean and dry. Finally, the valve on another coating product circuit is opened to fill the manifold and the pipes until a little of the new color coating product exits the sprayer. A new paint spraying phase may then begin.
If the installation contains only a small number of different color coating products, say three or four, the manifold may be located relatively near the sprayer. However, this solution is no longer practical if there are too many different coating products (there may be 20 or even more), and the color change unit must then be located remotely of the sprayer or sprayers. If the objects to be coated are large, as is the case with automobile bodies, the spraying means move over great distances parallel to the longest dimension of the objects, covering five or six meters, for example, especially in the case of robots. In this case the color change unit is very remote from the sprayer or sprayers, which considerably increases the volume of pipes to be cleaned and therefore the duration of the color change sequences. Because of this the quantities of coating product and cleaning product wasted on each color change cycle are large, in the order of several hundred cubic centimeters.
In an electrostatic installation all these color change related problems are further accentuated if coating products which have a low electrical resistivity, as do water-based paints, for example, are to be used without losing the advantages of electrostatic spraying. In this case the sprayer is often raised to a high-tension voltage and it is necessary to avoid short-circuiting between the sprayer and the coating product circuits which are electrically grounded. To counter the specific problem of changing color with low resistivity coating products, French patent No 2 572 662 (and the corresponding United Kingdom patent application No 2 166 982A published 5-21-86) proposes to fill an intermediate storage tank with just the sufficient quantity of paint for each application. The arrangement is such that this intermediate storage tank is grounded while it is being filled and at the high-tension voltage when it is feeding the sprayer or sprayers. The same intermediate storage tank is used for all colors, with the result that each color change makes it necessary to clean the storage tank and also all the pipes connected to it.
Also, it is relatively difficult in this type of installation to adjust the quantity of coating product to be fed into the intermediate storage tank. This quantity must be sufficient for the entire object to be painted. However, any excess is lost on rinsing which leads to a reduction in the overall efficiency of deposition and may result in the loss of all or part of the benefits obtained from electrostatic application. The invention makes it possible to overcome these problems.